This invention relates in general to error rate detectors, and in particular to error rate detectors for use in digital transmission equipment.
In digital transmission, the quality or reliability of the transmission system may be expressed in terms of an error rate, i.e., the number of erroneously received digital pulses during a predetermined time interval, which interval is set by the number of bits which are to be error-free and the bit rate. For example, if a 1 in 10.sup.-6 error rate is to be desired, then only one error in every one million error opportunities would be permitted. If there is more than one error in every one million opportunities, an error rate detector should identify this situation by producing an alarm indication.
Many of the techniques used in the prior art are based upon the use of a test signal which is transmitted during an out of service condition. Most often the receiving terminal is arranged to generate a replica of the test signal once the use of a test signal has been identified by the receiver and the test text from the transmitter is synchronized with the replica of the test text locally generated by the receiver. Errors are detected by making a comparison between the binary signal reconstructed at the receiver with that of the replica generated at the receiver. This technique is exemplified by the teaching of U.S. Pat. No. 3,725,860, entitled "Process And Circuit Arrangement For The Measuring Of The Frequency Of Bit Errors And Block Errors With Optional Block Length In The Transmission Of Binary Coded Data Characters," granted Apr. 3, 1973 to J. Kemper et al. As the patentee points out, the measurement of the frequency of error ocurrences uses a standardized test text having a fixed cycle length, such as is recommended by CCITT. This recommended text is pseudo-random text having a length of 511 bits (2.sup.-9 -1 bits). The simplest solution of the measurement of the frequency of block errors results with the standard 511 bit test text, when one selects a block length of 511 bits. The patentee notes that in practice in data transmission systems it is often desirable to use different block lengths than the standard 511 bit length. Therefore, it is necessary in the measurement of the quality of transmission to carry out the measurement of the frequency of errors with different block lengths. This is one of the techniques solved by the patentees and appears to be accomplished by relating the block length to the text in such a way as to be able to obtain block lengths which are different from the standard. Each error whether a single bit or a block error is read directly into an error counter and if the count exceeds that desired for the error rate the error counter will provide an output to an alarm circuit. In contrast, the instant invention teaches a technique for the detection of an error rate during an in service condition. Therefore, a fixed text such as is taught in the referenced patent cannot be used. Further, the error counting arrangement cannot be designed to simply count the errors during the testing period, since the test period is continuous.